When building external modules, $(objtree)/Module.symvers is scanned
for symbol information of vmlinux and in-tree modules.

Additionally, vmlinux is parsed if it exists in $(objtree)/.
This is totally redundant since all the necessary information is
contained in $(objtree)/Module.symvers.

Do not parse vmlinux at all for external module builds. This makes
sense because vmlinux is deleted by 'make clean'.

'make clean' leaves all the build artifacts for building external
modules. vmlinux is unneeded for that.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>